An aircraft comprises a skin which envelops the structure of the aircraft and which is in contact with the airflow in which the aircraft is moving. Such a skin has numerous elements which are also plunged into the airflow. These elements are for example cavities formed in the skin or additions such as for example antennas which then protrude.
Because of the airflow, these different elements generate sound signals, in particular whistling.
In the case of a cavity, these sound signals are due to the impact on the downstream edge of the cavity of the vortices which are generated at the upstream edge. At the location of this impact, a deceleration and a deformation of the vortices take place which create an acoustic wave and which gives rise:                either to an oscillation of the fluid in the cavity which excites one of the modes of the cavity and the whistling frequency is then dictated by the shape of the cavity,        or to a self-sustaining oscillation of the shear layer of the airflow which passes above the cavity since, at the time of the impact, the acoustic wave created goes back along the cavity and excites the vortices emitted at the upstream edge, and therefore the acoustic intensity at the upstream edge and so on.        
In order to eliminate the sound signals thus generated, it is known to add devices which prevent the creation of the phenomena which induce the whistling. These devices are for example vortex generators installed upstream of the cavities or deflectors which shift the reattachment point beyond the cavity.
Even though these various solutions are satisfactory from the reduction of sound signals point of view, they necessitate the installation of devices on the skin and these devices add to the weight of the aircraft and potentially increase the drag of the aircraft.